It's a question that no parent or friend wants to answer in the affirmative, but it turns out that, with normal-weight teens, this reluctance may make sense: A new Norwegian study shows that those who think they're too heavy are the ones most likely to be overweight by early adulthood.

In what is described as the first study to capture the relationship between perceived and actual weight over a period of several years, researchers said they found that a negative self-image may play a key role in driving teens toward obesity.

Among both girls and boys who were normal weight as teens but felt fat, 59 percent of the girls became overweight as women and 63 percent of the boys became overweight as men.

"When you experience yourself as overweight, it raises your chances of actually becoming overweight," said study author Koenraad Cuypers, a researcher at the Norwegian University of Science and Technology, in Trondheim.

"We think it has to do with the idealization of their bodies, seeing all the models in the world of clothes and in the movies," Cuypers explained. "You have to be thin to be successful. That is the ideal."

So how does that actually translate into gaining pounds rather than staying trim?

"It's stressful. Once you think you're fat, you'll feel fat in adulthood," Cuypers said. "Feeling fat may create a kind of psychosocial stress because they can't reach the look they think they have to have."

The researchers also speculate that those who went into adulthood feeling fat may have repeatedly tried to lose weight, which some studies suggest can increase the tendency to become overweight. Others may have tried skipping meals, which this study's data suggest can widen the gap between real and imagined weight.

According to the World Health Organization, more than 1.4 billion adults were overweight as of 2008. Of these, more than 200 million men and nearly 300 million women were obese. Excess pounds can contribute to a wide range of diseases and conditions, including heart disease and stroke, diabetes, musculoskeletal problems such as arthritis, and some cancers.

The health survey included 1,196 normal-weight teenagers of both sexes. Data were collected from 1995 to 1997, and again in 2006 to 2008, when the participants were between 24 and 30 years of age. The researchers excluded those with physical or mental disorders and those who were pregnant. They also adjusted for pubertal timing and the participant's level of physical activity.

The study could help guide health education approaches about food and self-image in teens, said S. Bryn Austin, an associate professor in the department of society, human development and health at the Harvard School of Public Health. "It's an excellent study that highlights that when kids feel bad about their bodies they take less good care of them," she said.

Austin said parents should avoid telling teenagers they look fat or need to lose weight, and focus on creating a healthy, physically active environment at home. "Serve good food, offer a fun, physically active lifestyle, take the TVs out of the bedrooms," she advised. "Don't focus on restrictions or fear of weight gain."

She also blames the media for focusing too much on the obesity epidemic. "Body dissatisfaction is part of our obesigenic environment," she said. "There's a lot of hyper-focusing on the dangers, which increases dissatisfaction, adds stress and contributes to unhealthy attitudes and behaviors," she said. "It's a perfect storm."